debuginfod: User-Agent and X-Forwarded-For header relay

debuginfod: User-Agent and X-Forwarded-For header relay

Extend the debuginfod client API with a function to stuff outgoing
headers into libcurl http transfers.  Use this from debuginfod so
federated trees of debuginfod/httpd can trace back to to the
originating client for administrative purposes.  Docs & test included.

Signed-off-by: Frank Ch. Eigler <fche@redhat.com>